Reduced capacitance damper and method

ABSTRACT

A magneto rheological (MR) fluid damper system for use in a vehicle suspension system can include a twisted pair of conductors integrated into a hollow piston rod of the MR damper. The twisted pair of conductors provide a supply path for current to reach the actuating coil of the MR damper while also providing the current return path to an electric source. The twisted pair of conductors facilitates a reduction in capacitance in the operating circuit, and thus minimizes electromagnetic interference affecting other electrical systems in the vehicle.

BACKGROUND

1. Field

The presently disclosed subject matter relates to a Magneto Rheological(MR) fluid damper system for use in a vehicle suspension system and itsmethod of use. More particularly, the disclosed subject matter relatesto an MR damper where electromagnetic interference (EMI) is reduced byreducing capacitance in the damper. For example, the conventional artcurrent path that includes an integral return path through the damperitself can be replaced with a twisted pair of conductors.

2. Brief Description of the Related Art

Vehicle suspension systems utilize damping devices or shock absorbersfor controlling the vibrations of the body and wheel due to roaddisturbances imposed on the mass-spring system of the vehicle body,wheels and suspension springs. A vehicle suspension damper usuallyprovides a resistive force proportional to the relative velocity betweenthe body and the wheel. Passive dampers may employ an oil-filledcylinder and piston arrangement, while active and controllable dampersmay employ a magneto-rheological (MR) fluid-filled cylinder and pistonarrangement where the viscosity of the MR fluid may be changed by theintroduction of a magnetic field which in turn varies the amount ofdamping provided by the MR damper.

In recent years, hydraulic dampers have been utilizing MR fluid as partof vehicle suspension systems as vibration dampers. MR fluids respond toa magnetic field with a dramatic change in rheological behavior. Thesefluids can reversibly and instantaneously change from a free-flowingliquid to a semi-solid with controllable yield strength or therebetweenwhen exposed to a magnetic field. In the absence of an applied field, MRfluids are reasonably well approximated as Newtonian liquids, but whenan electric field is applied, the fluid becomes viscous and theviscosity increases as the potential of the electric field increases.One of the characteristics of MR fluid is that it changes its state veryrapidly when an electric field is applied or released. This featuremakes MR fluid a suitable element in hydraulic dampers. For mostengineering applications, a simple Bingham plastic model is effective indescribing the essential, field-dependent fluid characteristics.

Vehicle suspension systems that control damping by means of electricpower, such as electronically powered actuators, generate various levelsof conducted electromagnetic interference (EMI) or noise, which couldinterfere with other electrical devices in a vehicle such as radios andthe like. This noise needs to be either contained or reduced. Noise is aproblem that needs to be addressed, and several U.S. patents haveattempted to solve this problem with respect to conventional artdevices. These U.S. patents, listed below, are hereby incorporated intheir entirety into this disclosure by reference.

For example, U.S. Pat. No. 6,926,288 (the '288 patent) discloses anelectromagnetic interference filter for use in reducing electrical noiseproduced by an electrically actuated vehicle suspension system. Inparticular, the '288 patent discloses the use of a filter board thatincludes capacitors and inductors. The filter board is attached inseries with the power switching mechanism for the electrically actuatedvehicle suspension system such that noise generated by the switchingmechanism can be suppressed from traveling back to the DC power source.Thus, the filter board of the '288 patent also prevents noise frominterfering with the radio or other electrical components of thevehicle.

Similarly, U.S. Pat. No. 5,392,881 discloses a device for dampeningvibratory motion that uses a thick housing for insulating otherelectronic equipment from the noise of the damper actuation.

There are several patents that utilize MR dampers, such as U.S. Pat.Nos. 6,637,556; 5,014,829; and 6,547,044. As shown in FIGS. 3 a and 3 bof the present application, one conventional type of MR damper includesa piston assembly 928 disposed in the damper body tube 914 which formsan annular flow gap 960 between the piston assembly 928 and the damperbody tube 914. The piston assembly 928 is connected to a hollow pistonrod 930 and is fixed in position within the housing tube 914. The pistonassembly 928 has a piston core 946 mounted on one end of the piston rod930 that is formed of a ferromagnetic material. The piston furtherincludes a magnet assembly 948 including a coil 950 mounted on thepiston core 946. The coil 950 is connected to an electrical source (notshown) via an electrical connector extending through the piston rod 930.The piston assembly 928 divides the volume of MR fluid within the damperbody tube 914 into a compression chamber 964 and an extension chamber966.

The current path to the coil 950 flows via an electrical connector 956located within the piston rod 930. Typically, the piston rod 930 itselfor possibly the sleeve 988, the tube 914, or other integral structureserves as the current return path.

The MR fluid within the damper body typically includes micron-sizedferrous particles dispersed in a fluid or an elastomer. When the MRfluid is exposed to a magnetic field, the liquid state is changed tosemi-liquid or to a solid state. The speed of rheology is on the orderof a millisecond. The yield stress of the semi-solid material has arelationship with magnetic field density. A desired damping effectbetween the sprung and unsprung masses of the vehicle is achieved bycontrolling the application of an electric current from the electroniccontrol unit (ECU). The electrical current dictates the MR fluid's yieldstress, which in turn determines damping resistance.

These types of MR dampers are well-suited to provide a controllabledamping effect for the suspension system of a vehicle. One problem isthat this kind of damper inherently contains parallel capacitance (dueto the existence of the rod and the electrical conductor) which acts asa temporary short circuit, causing a large electrical current spike totravel down the control line, which acts as noise. This phenomenonoccurs when the rod or other integral structure is used as a return pathfor the electrical current from the coil. By reducing capacitance in thedamper, this noise can be reduced. One way to solve this problem is toadd inductance to the ECU in series with the damper. However, thissolution is expensive and introduces significant weight to the ECU.

However, the EMI problem, the weight problem, spacing problems, andother considerations have not been addressed in the aforementionedconventional art patents. Accordingly, there exists a need for animproved MR damper device in which the EMI is reduced, weight isminimized, spacing requirements are minimized, and other features andbenefits are achieved that will be apparent to one of skill in the artfrom a reading of the description of the disclosed subject matter below.

SUMMARY

In accordance with one aspect of the disclosed subject matter, a magnetorheological (MR) damper can include a cylinder tube that has a workingchamber and a rebound chamber. The working and rebound chambers caninclude an MR fluid whose viscosity is controllable via a pulse widthmodulated current provided by a controller. A piston assembly can beattached to a hollow piston rod and housed within the cylinder tube. Aflow gap can be formed between the piston assembly and the cylindertube. Furthermore, the piston assembly can include a magnetic core and aflux ring positioned around the magnetic core. The flux ring provides aseal within the cylinder tube to separate the MR fluid in the workingchamber from the MR fluid in the rebound chamber. A twisted pair ofelectrical conductors can be housed in the hollow piston rod. A coil canbe mounted on the magnetic core for generating a magnetic field, whereinthe coil is operatively interconnected to an electrical source via thetwisted pair of electrical conductors extending through the piston rod.The twisted pair of electrical conductors provides an electrical returnpath to the electrical source.

In accordance with another aspect of the disclosed subject matter, thereis provided a magneto rheological (MR) damper for reducing capacitancein a vehicle suspension system including a cylinder tube that has aworking chamber and a rebound chamber. The working and rebound chamberscontain an MR fluid. A piston assembly can be attached to a hollowpiston rod and housed within the cylinder tube, wherein a flow gap isformed between the piston assembly and the cylinder tube. The pistonassembly can include a magnetic core and a flux ring can be positionedaround the magnetic core. The flux ring provides a seal within thecylinder tube to separate the MR fluid in the working chamber from theMR fluid in the rebound chamber. A twisted pair of electrical conductorscan be housed in the hollow piston rod. A coil can be mounted on themagnetic core for generating a magnetic field, wherein the coil isoperatively interconnected to an electrical source via the twisted pairof electrical conductors extending through the piston rod.

In accordance with still another aspect of the disclosed subject matter,the twisted pair of electrical conductors is connected to a 20 Hz FETdriver.

In accordance with still another aspect of the disclosed subject matter,the magnetic field produced by the twisted pair of conductors duringoperation of the damper produces noise that is less than substantially20 dBμV/539 KHz.

In accordance with still another aspect of the disclosed subject matter,there is provided a method of reducing capacitance and/or reducingelectrical noise in a MR damper that can include passing an electricalcurrent from the electrical source through the twisted pair ofconductors extending through the hollow piston rod to the coil.

In accordance with another aspect of the disclosed subject matter, amagneto rheological damper system can include a working chamber, arebound chamber in fluid communication with the working chamber, and anMR fluid located in at least one of the working chamber and the reboundchamber. A fluid communication structure configured to communicate theMR fluid between the working chamber and the rebound chamber can also beprovided. A piston assembly including a piston and a hollow piston rodcan be operatively connected to the piston, and a magnetic core can belocated adjacent the fluid communicating structure. A coil can belocated adjacent the magnetic core, and a twisted pair of electricalconductors can be located in the hollow piston rod and electricallyconnected to the coil to provide both a current supply path and currentreturn path to and from the coil. Application of a current to the coilthrough the twisted pair of conductors can cause the coil to generate amagnetic field that affects the viscosity of the MR fluid.

In accordance with yet another aspect of the disclosed subject matter,the MR damper hollow piston rod can include an insulator materialtherein.

In accordance with yet another aspect of the disclosed subject matter,the fluid communication structure can be configured as a flux ringpositioned around the magnetic core, and the flux ring can separate theMR fluid in the working chamber from the MR fluid in the reboundchamber.

In accordance with yet another aspect of the disclosed subject matter,the fluid communication structure can be integrated into the piston ofthe piston assembly.

Still other features and attendant characteristics of the disclosedsubject matter will become apparent to those skilled in the art from areading of the following detailed description of embodiments constructedin accordance therewith, and taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed subject matter will now be described in more detail withreference to exemplary embodiments of the apparatus and method, givenonly by way of example, and with reference to the accompanying drawings,in which:

FIG. 1 a is a cross sectional representation of an embodiment of a MRdamper made in accordance with principles of the disclosed subjectmatter;

FIG. 1 b is an enlarged cross sectional representation of a portion ofthe MR damper of FIG. 1 a;

FIG. 2 a is a diagram illustrating the electrical current loop in aconventional MR damper;

FIG. 2 b is a diagram illustrating the electrical current loop in an MRdamper according to the present disclosed subject matter;

FIG. 3 a is a cross sectional representation of a related art MR damper;and

FIG. 3 b is an enlarged cross sectional representation of a portion ofthe related art MR damper of FIG. 3 a having the rod as the return pathfor electric current from the damper coil back to the ECU.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to the drawing figures, like reference numerals designateidentical or corresponding elements throughout the several figures.

FIG. 1 a is a cross sectional representation of an MR damper made inaccordance with principles of the disclosed subject matter. FIG. 1 b isan enlarged cross sectional representation of a portion of the MR damperof FIG. 1 a. The MR damper can include a cylinder tube 1 that defines acompression chamber 22 and rebound chamber 5 for containing MR fluid. Apiston assembly 2 can be attached to a hollow piston rod 17 and fixed inposition within the cylinder tube 1. The piston assembly 2 includes amagnetic core 9, a flux ring 10, and a coil 8 mounted on the core 9. Thepiston assembly 2 divides the volume of MR fluid within the damper intothe compression chamber 22 and rebound chamber 5, and acts to meter outthe MR fluid between the chambers during operation. The coil 8 isconnected to an electrical source via a twisted pair of conductors 4extending through the hollow piston rod 17. The electrical power/signalsource can be provided by an Electronic Control Unit (ECU) 51 locatedwithin the vehicle.

In the embodiment shown in FIG. 1 b, the flux ring 10 positioned aroundthe core 9 and includes a flow gap 15. The MR fluid can includemicroscopic particles of a magnetic material suspended in a liquidcarrier. When the MR fluid is exposed to a magnetic field of sufficientstrength, the suspended particles align with the magnetic field andcause a change of viscosity in the MR fluid. Thus, by controlling theapplication of electric current to the coil 8, the magnetic field isvaried and the flow characteristics of the MR fluid are also affected.Thus, the flow of MR fluid through the flow gap 15 can be controlled ina predictable manner. In particular, the ECU 51 can be used to controlthe current supplied to the coil 8, which in turn creates a magneticfield to control the viscosity of the MR fluid. The ECU 51 can use apulse width modulated signal at 20 KHz, for example, to control thecurrent that is provided to the coil 8.

When large vibrations are sensed during travel of a vehicle, the ECU 51can provide a current amount that causes the MR fluid in the MR damperto become more viscous, and therefore slow the fluid flow through theflow gap 15, and likewise slow the relative movement between the piston2 and the damper tube 23. This action provides an effective andcontrolled damping to the vehicle.

During operation of the MR damper, electric current is supplied to thecoil 8, wherein such application of electric current generates amagnetic field, as described above. The twisted pair of conductors 4provides both the supply flow path and return path for current, suchthat electric current flows from the ECU, through the coil 8, and backto the ECU via the twisted pair of conductors 4. By using the twistedpair of conductors 4, the load capacitance in the damper can besignificantly reduced as compared to conventional damper mechanisms, andthus the electromagnetic interference (i.e., noise) in the control lineis reduced without requiring added inductance and its attendant spaceand weight requirements.

The twisted pair of conductors 4 may be wires constructed of steel,copper, aluminium, combinations of such materials, or other electricallyconductive materials. The conductors 4 may optionally include insulatormaterial, such as plastic, vinyl, or the like.

FIGS. 2 a and 2 b are diagrams illustrating a conventional current loopin an MR damper (FIG. 2 a) as compared to an example of a current loopin an MR damper made in accordance with principles of the disclosedsubject matter (FIG. 2 b). The conventional art employs a singleelectrical conductor located in the piston rod and routes the returncurrent path through the piston rod itself. In contrast, the damper asshown in FIG. 3 b employs a pair of twisted conductors that provide boththe current supply path and return path to and from a coil. Onecharacteristic of the twisted pair of conductors is a reduction incapacitance as compared to the conventional art. This can be shown bythe formula for capacitance:

C=εA/d,

-   -   where C=capacitance,    -   ε=permittivity,    -   A=area for plate overlap, and    -   d=distance between the plates.

As shown in FIG. 2 b, an electric current from the ECU is conductedthrough the damper via a twisted pair of conductors. The current isreturned to the source through the same twisted conductors. Thus,because the “plate overlap area” (A) is small for a twisted pair ofconductors, the capacitance can be minimized. In addition, insulatingcovers can be provided on each conductor of the twisted pair ofconductors to provide a distance (d) between each of the conductors suchthat, combined with the minimal plate overlap area A, capacitance isfurther minimized. In addition, theoretically, the twisted conductorsshould produce little or no interference and cancel out each other'snoise characteristics due to their twisted nature. All of these factorstaken in combination help to minimize the capacitance and the resultantnoise generated by the electrical circuit for the MR damper.

Although certain embodiments of the disclosed subject matter aredescribed above, it should be understood that the disclosed subjectmatter can be embodied and configured in many different ways withoutdeparting from the spirit and scope of the invention. For example, thefluid communication structure that regulates the amount and speed of MRfluid flow between the compression chamber and the rebound chamber canbe formed on other structures besides the piston assembly. Inparticular, the cylinder wall can be provided with a fluid communicationstructure that has a flow gap to regulate the transfer of MR fluidbetween the compression chamber and the rebound chamber. In addition, astructure that is separate from both the piston assembly and cylindercould conceivably be used in an MR damper in which the compressionchamber and rebound chamber are formed above the piston assembly or inanother manner. In addition, the flux ring can include or comprise thefluid communication structure that communicates between the compressionchamber and rebound chamber.

Alternate means for applying the magnetic field to the MR damper arecontemplated. For example, a plurality of coils and/or cores can beprovided to achieve the desired magnetic field onto the MR fluid.

The compression chamber and rebound chamber need not be above and belowthe piston as shown in the various exemplary figures, but could bearranged such that they both reside on one side of the piston, forexample. In this case, the piston would push the MR fluid in to areservoir style rebound chamber that receives MR fluid duringcompression and then drains the MR fluid back to the compression chamberduring rebound.

The cylinders and other components of the MR damper can be made frommetals, plastics or even ceramic materials. The cylinder materials, suchas the ceramics materials, can further enhance the reduction incapacitance and resultant noise characteristics of the MR damper.

While the disclosed subject matter has been described in detail withreference to exemplary embodiments thereof, it will be apparent to oneskilled in the art that various changes can be made, and equivalentsemployed, without departing from the scope of the invention. Each of theaforementioned conventional art documents is hereby incorporated byreference herein in its entirety.

1. A magneto rheological (MR) damper comprising: a cylinder including aworking chamber and a rebound chamber, wherein at least one of theworking chamber and rebound chamber includes an MR fluid; a hollowpiston rod; a piston assembly operatively connected to the hollow pistonrod and housed within the cylinder, wherein a flow gap is formed by oneof the cylinder and the piston assembly, and wherein the piston assemblyincludes, a magnetic core, and a coil located adjacent the magnetic coreand configured to generate a magnetic field upon application of anelectric current to the coil; and a twisted pair of electricalconductors housed in the hollow piston rod, wherein the coil isconfigured to be connected to an electrical source via one of thetwisted pair of electrical conductors that extend through the pistonrod, and the other of the twisted pair of electrical conductors providesan electrical return path from the coil to the electrical source.
 2. TheMR damper of claim 1, further comprising: a flux ring positionedadjacent the magnetic core, wherein the flux ring provides a seal withinthe cylinder to separate the MR fluid in the working chamber from the MRfluid in the rebound chamber.
 3. The MR damper of claim 1, wherein thehollow piston rod includes an insulator material therein.
 4. A method ofreducing capacitance in a magneto rheological (MR) damper according toclaim 1 comprising: passing an electrical current from the electricalsource through the twisted pair of conductors extending through thehollow piston rod to the coil.
 5. A method of reducing electrical noisein a magneto rheological (MR) damper according to claim 1 comprising:passing an electrical current from the electrical source through thetwisted pair of conductors extending through the hollow piston rod tothe coil.
 6. A magneto rheological (MR) damper system, comprising: aworking chamber; a rebound chamber in fluid communication with theworking chamber; an MR fluid located in at least one of the workingchamber and the rebound chamber; a fluid communication structureconfigured to communicate the MR fluid between the working chamber andthe rebound chamber, a piston assembly including a piston and a hollowpiston rod operatively connected to the piston; a magnetic core locatedadjacent the fluid communicating structure; a coil located adjacent themagnetic core; and a twisted pair of electrical conductors located inthe hollow piston rod and electrically connected to the coil to provideboth a current supply path and current return path to and from the coil,wherein application of a current to the coil through the twisted pair ofconductors causes the coil to generate a magnetic field that affects theviscosity of the MR fluid.
 7. The MR damper system of claim 6, whereinthe hollow piston rod includes an insulator material therein.
 8. The MRdamper system of claim 6, wherein the fluid communication structure isconfigured as a flux ring positioned around the magnetic core, and theflux ring separates the MR fluid in the working chamber from the MRfluid in the rebound chamber.
 9. The MR damper system of claim 6,wherein the fluid communication structure is integrated into the pistonof the piston assembly.
 10. The MR damper system of claim 9, wherein thecoil and magnetic core are integrated into the piston of the pistonassembly.
 11. The MR damper system of claim 6, wherein the coil andmagnetic core are integrated into the piston of the piston assembly. 12.A method of reducing electrical noise in a magneto rheological (MR)damper according to claim 6 comprising: passing an electrical currentthrough the twisted pair of conductors extending through the hollowpiston rod to the coil.